The רמב’’ם brings the law about a lender and borrower that disagree about the value of a pledge for the loan that as lost through not fault. in that case [מלווה ולווה פרק י'ג הלכה ד'] the lender takes an oath that he does not have the pledge anymore, the borrower takes an oath about its value and pays what he admits beyond the value of the pledge. The רמב''ן (ר' משה בן נחמן)asks let him tell the lender, "Give me back my pledge, and I will repay the debt." רב שך suggests an answer to this question. He says after that the תורה requires one to take an oath, the other party can not claim he does not believe him. My question is based on the law about a פיקדון. When the owner asks for it back, and it is denied, רש''י says the keeper’s oath: "It is no longer in my possession" is from the תורה. The רמב’’ם holds the oath in that case is דרבנן. So, in our case also, if we assume a pledge is like a deposit, then the רמב’’ם would be ok if we would go like רש''י. But the רמב’’ם holds the oath for a deposit is דרבנן , and perhaps a משכון too. The claim of the borrower that the lender might still have the pledge might still be relevant. [I recall רב שך going into this somewhere in the אבי עזרי but I cannot recall where.]